It is often desirable to reinforce rubber articles by incorporating therein metal reinforcing elements. For example, tires, conveyor belts, power transmissions belts, timing belts, hoses, and a variety of other rubber articles are often reinforced with metal elements. In order for rubber articles which are provided with steel reinforcing elements to function effectively it is imperative that good adhesion between the rubber and the metal reinforcing element be maintained.
Methods of improving metal to rubber adhesion have been the subject of considerable experimentation and research. Various solutions have been suggested which have provided various degrees of success. For example, various physical configurations of cabled wire filaments have been used to enhance physical or mechanical adhesion to rubber. Also, the surface of wire filaments have been treated by various materials and methods to enhance their adhesion to rubber. For example, pneumatic vehicle tires are often reinforced with cords prepared from steel filaments which are coated with brass.
Steel is very prone to oxidation, which even in minor degrees is highly deleterious to rubber-metal adhesion. Thus, generally steel reinforcement elements are coated with brass in order to facilitate rubber-metal adhesion. Normally steel reinforcing elements are coated with brass that is an alloy of only copper and zinc. However, ternary brass alloys that are useful for coating steel reinforcing elements are known by those skilled in the art. For example, U.S. Pat. No. 4,347,290 discloses a ternary brass alloy containing copper, zinc, and cobalt. Coating steel reinforcing elements with ternary brass alloys containing copper, zinc, and iron is also known to be effective in improving rubber to metal adhesion. Steel reinforcing elements can be provided with work-hardened nickel coatings in order to improve the steels resistance to static fatigue corrosion. U.S. Pat. No. 3,749,558 discloses a steel tire reinforcement coated with a layer of nickel which is covered by a brass outer coating. Thus, it is known to coat metal reinforcing elements with various alloys to improve the reinforcing elements adhesion to the rubber in which it is embedded. It is also known to treat metal reinforcing agents with various chemicals in order to improve their adhesion to rubber. For instance, U.S. Pat. No. 3,586,568 discloses a process for treating such a metal reinforcing element with a mixture of chromic acid and phosphoric acid in order to improve the metals bonding to elastomeric materials. U.S. Pat. No. 4,299,640 reveals that the adhesion of brass plated steel cords to rubber can be improved by treating such cords with dilute aqueous solutions of certain amino carboxylic acids and their corresponding ammonium, lithium, sodium, and/or potassium salts and salt hydrates.
It is also known that various agents can be mixed into rubber which will increase the adhesion between the rubber and metal reinforcements embedded in it. U.S. Pat. No. 3,894,903 discloses a process for improving the bonding of rubber to copper and copper alloys by incorporating into the rubber before vulcanization certain s-triazines, for instance 2-m-hydroxyphenoxy-4-chloro-6-aminotriazine.
U.S. Pat. No. 3,991,130 discloses a method for improving adhesion between vulcanizable elastomeric compositions and metal surfaces by incorporating into the elastomer an organo-nickel salt and then subsequently vulcanizing the elastomeric composition while it is in contact with the metal surface. U.S. Pat. No. 3,676,256 indicates that metal reinforced rubber vulcanizates containing air oxidized furnace carbon blacks show improved adhesion to brass surfaces.
The materials to which this invention is directed, rubbers reinforced with metals, are within the general class of materials called composites. A composite is a complex material containing two or more distinct and structurally complimentary substances (in this case rubber and metal) which are combined to produce desirable structural and/or functional properties not present in any individual component. For example, steel belts can be incorporated into a pneumatic rubber tire to attain desirable properties that cannot be achieved if the tire is built with rubber alone. In order for the metal and rubber to compliment each other in such composites it is necessary for there to be good adhesion between the rubber and metal.